The present invention relates to utilizing a shape memory material in conjunction with hydraulic force conversion.
There are many varieties of shape memory materials including, for example, shape memory alloys (metals), shape memory polymers and shape memory proteins. Shape memory alloys have been known for years, while shape memory polymers and shape memory proteins are more recent developments. Research and development in the area of new or improved shape memory materials is an ongoing project in many different laboratories. Although different shape memory materials have different degrees of behavior and are actuated by differing environmental conditions, shape memory materials all have the unusual property of having a mechanical memory. If the shape memory material is given a first configuration and, if necessary, subjected to an appropriate treatment, and thereafter this configuration is deformed, the shape memory material will retain that deformed shape until the shape memory material is subjected to a predetermined environmental condition, such as, for example, an elevated temperature, an electrical current or a different pH environment, at which time it will return to its original configuration.
While returning to its original configuration, the shape memory material can perform work. In general, the change in environment (heat, electricity, pH, etc.) is all that is necessary to induce a high-stress recovery to the original predeformation configuration. In many cases, there is considerable mechanical stress output from the shape memory material while it is returning to its original predeformation configuration. Values of recovery stress in excess of 110,000 psi have been reported for certain shape memory alloys. Although each of the various different shape memory materials allow for mechanical shape recover, the shape memory alloys presently are the most widely used for high stress recovery.
Shape memory alloys have been known and available for many years and have been proposed as operative elements in various types of devices. Because of their dramatic strength and response to temperature, shape memory alloys have been proposed as alternatives to motors, solenoids, expandable wax actuators, and bimetallic temperature sensitive actuators. Although not a panacea, a shape memory alloy approach to electromechanical actuation may offer advantages which conventional approaches would find difficult or impossible. For example, large amounts of recoverable strain available from shape memory alloys offer work densities many times higher than conventional approaches. Also, the high electrical resistivity of shape memory alloys permits direct electrical actuation without extra parts and with efficient use of available energy.
Generally, the shape memory alloy is a nickel-titanium alloy called Nitinol or Tinel, although copper-based alloys have been used in many similar applications. Early investigations on Nitinol started in 1958 by the U.S. Naval Ordinance Laboratory which uncovered a new class of novel nickel-titanium alloys based on the ductile intermetallic compound TiNi. These alloys were subsequently given the name Nitinol which is disclosed in U.S. Pat. No. 3,174,851 of Mar. 23, 1965 entitled "Nickel-Based Alloys," U.S. Pat. No. 3,351,463 of Nov. 7, 1967 entitled "High Strength Nickel-Based Alloys" and U.S. Pat. No. 3,403,238 of Sept. 24, 1968 entitled "Conversion of Heat Energy to Mechanical Energy," all patents being assigned to the United States of America as represented by the Secretary of the Navy.
The great interest in the near stoichiometric TiNi composition alloys stems from their unusual mechanical memory. This thermal-mechanical shape memory, or shape memory effect, allows a shape memory alloy like Nitinol to return to a preset shape after mechanical distortion. If the shape memory alloy is given a first shape or configuration and subjected to an appropriate treatment, and thereafter its shape or configuration is deformed, it will retain that deformed shape or configuration until such time as it is subjected to a predetermined elevated temperature. When it is subjected to the predetermined elevated temperature, it tends to return to its original shape or configuration. Heating above the predetermined elevated temperature is the only energy input needed to induce high-stress recovery to the original predeformation shape. The predetermined elevated temperature is usually referred to as the transition or transformation temperature. The transition or transformation temperature may be a temperature range and is commonly known as the transition temperature range (TTR).
As is well known, a shape memory alloy has two states, separated only by temperature. When cooled, the shape memory alloy is in the martensitic state, in which the alloy is relatively soft and easily deformed. When warmed above the TTR, the shape memory alloy is transformed into the austenitic state in which the alloy is much stronger and stiffer than when in the martensitic state. When in the martensitic state, the alloy may be deformed or changed in configuration from a preset configuration while under relatively low load. When the alloy is heated through its TTR, the alloy remembers its original preset shape and tends to return to that shape. In the process, it builds up forces that oppose the deformation which occurred in the martensitic state, and the alloy can perform work while returning to its original shape.
Shape metal alloys have previously been used for actuator-type devices, often using elongated wire-shaped lengths of the alloy in tension (straight sections of wire) or in a combination of torsion, tension, and compression (helical coils of wire). The shape metal alloy wire is deformed while cool. When activation is required, the wire is heated to a temperature above the TTR, usually by passing an electric current through it. High electrical resistivity (similar to nichrome) of the shape metal alloy wire allows such an electrical current to impart thermal energy evenly along the length of the wire.
A metallurgical phenomenon which enables Nitinol alloys to have "shape memory" has been proposed, although the exact mechanism of energy exchange within the shape memory alloy is still in debate. The high temperature phase of the Nitinol is a body-centered-cubic crystal structure, usually referred to as an austenite. The low temperature phase of Nitinol is a twinned martensite which is represented by slightly shifting alternate rows of atoms away from the perpendicular registry of the higher temperature austenite phase. If the Nitinol in the twinned martensitic state is allowed to warm through its transition temperature range (TTR), it must return to the austenitic state. Since the state change is diffusionless, the transformation of Nitinol from the martensitic to the austenitic structure occurs very rapidly over a narrow temperature range. When a Nitinol specimen is cooled, it transforms from its austenitic state to a twinned martensitic state. While in this state, the specimen can be easily deformed by the application of a stress, which eliminates the martensitic twin. The applied stress shifts the alternating atomic registry of the twinned martensitic structure to a parallel registry. The deformation of the twinned martensitic specimen, resulting in the atoms slipping to a new parallel position, is a deformation which will be recovered upon heating. Unlike all other heat-exchange systems, Nitinol responds to temperature changes in an unbalanced way, in that the force needed to bend it when it is cold is much less than the force it releases when it returns.
The narrow transition temperature range (TTR) over which the shape memory alloy recovers its shape is primarily a function of the alloy's composition, which is typically about 53% to about 57% Ni balance Ti. A third, interstitial element, such as cobalt, may also be added to the alloy to control the TTR temperature. A direct atom-for-atom substitution of cobalt for nickel is usually performed to progressively lower the TTR. The TTR at which the "shape memory effect" (SME) occurs may be set anywhere from -200.degree. C. (Liquid Nitrogen) to 150.degree. C. with great accuracy (.+-.1.degree. C.). For example, the TTR may be varied rather precisely as follows: a 1.0% change of the Ni/Ti ratio results in a 150.degree. C. change in the TTR or 70 ppm NiTi per 1.degree. C. It should also be noted that recoverable straining must be performed below the TTR of the shape memory alloy.
The shape memory effect covers three principal plastic deformation modes. These are (1) uniaxial tension, (2) torsion or twisting, and (3) bending (combined tension and compression). Compression, while a very useful mode, was not considered in the present invention because of the difficulty and complexity associated with its stressing and straining. However, the mode which utilizes the shape memory effect best volumetrically is the uniaxial tension mode, because the entire cross-section of the specimen is used for the shape memory effect. In fact, the highest recovery forces presently produced are induced under uniaxial tension with a 20-mil (0.020-inch) diameter wire.
Another unusual important property of Nitinol alloys is the amount of deformation or strain that can actually be recovered. If the motion or force is desired only once during the life of the alloy, large deformations or strains, such as 8% to 20%, can be utilized. If the desired motion or force requires repeated cycling during the lifetime of the alloy, it is important that the straining not exceed a critical level, or critical strain level (CSL), usually 6% to 8%, to insure recoverable plastic straining. Straining beyond this (CSL) limit will result in incomplete shape recovery of the shape memory alloy. Accompanying the shape recovery is a large energy conversion (heat to mechanical) which is capable of overtly exerting a large force or recovery stress. Values of recovery stress in excess of 110,000 psi have been reported for a 20-mil wire during uniaxial plastic straining of 6% to 8%. This recovery stress is proportional to the initial strain, and temperatures higher than the TTR are required to obtain maximum recovery stresses. The higher the initial strain, the greater the temperature difference between the TTR and maximum recovery stress temperatures. Also, as in the case of recovery stress, there is an optimum strain to obtain maximum work output. Values of maximum work output in excess of 2600 in-lbf/in.sup.3 have been reported for a 20-mil wire during uniaxial plastic straining of 6% to 8%. Overall, the shape recovery produces high stress and work output.
Nitinol also has high electrical resistivity (approximately 76 microhm-cm), similar to nichrome; thus, it permits direct electrical shape memory actuation via resistance heating. Such heating results in an efficient work output as well as efficient use of energy input. Furthermore, the resistance will change slightly with temperature, depending on whether the temperature is rising or falling (heating or cooling). This change is dependent on specimen temperature and specimen memory state. Thus, the alloy will also lend itself to resistive feedback monitoring.
Some other interesting properties and characteristics of Nitinol should be mentioned. First, the cycle life of Nitinol has been reported as approaching infinite (2.5.times.10.sup.7 cycles) when strained below the recoverable strain limit (6% to 8%), limited only by the extent of testing performed to date. Second, Nitinol can develop a secondary shape memory. A "2-way" shape memory can be programmed into Nitinol specimens by appropriately repeating stress and/or thermal cycling. Once this conditioning has been achieved, a specimen will spontaneously revert to a shape when cooled, as well return to the initial memory state when heated. Finally, Nitinol is virtually non-magnetic, and practically inert to harsh corrosive environments, due to its elemental makeup. This allows Nitinol to be used in a wide variety of industrial and corrosive environments.
Previous research and development of the Nitinol has clearly indicated the potential for actuator-type devices. Such criteria as uniformly reliable strain-heat-recovery, accurate composition-related recovery range, high force and work output, electrical controllability of recovery, extremely high fatigue life, corrosion resistance, and non-magnetic nature are desirable criteria for prime movers in actuation devices.
Repeatability is normally required in actuator applications. In this respect, it is desirable that the Nitinol part return to its deformed shape upon cooling (after the heating which changes the deformed shape to the memory shape), so that it can revert to its memory shape again in successive cycles. Since the yield strength of Nitinol is low at temperatures below the TTR, reversibility can be effected by biasing the Nitinol element with a common spring. When Nitinol is heated, it exerts more than a sufficient force to overcome the spring completely and perform the desired shape memory operation. On the other hand, as soon as the Nitinol part cools through its TTR, the spring is now strong enough to force the Nitinol back into the deformed shape. In this way, the Nitinol is ready to operate on the next heating cycle.
As stated previously, reversibility can be "built into" shape memory materials and alloys, so that the use of the biasing spring or similar devices is not always necessary. Once this reversibility has appropriately been conditioned into the alloy, the specimen will spontaneously revert to a shape when cold, without external biasing means.
A more recent and less widely known shape memory material is the so-called polymer or elastomer shape memory material developed by Dr. Daniel W. Urry. Use of shape memory polymers has been disclosed in several patents and patent applications, such as U.S. patent application Ser. Nos. 184,873, filed on Apr. 27, 1988 and 184,407, filed on Apr. 21, 1988. These two applications disclose the use of shape memory polymers in the medical field and are incorporated herein for their discussions of shape memory polymers and elastomers. These shape memory polymers and elastomers have not been configured, optimized or otherwise used as the prime mover for force converters or actuators but, when employed according to the disclosure herein, are suitable shape memory materials for the present invention.
U.S. patent application Ser. No. 184,873 also discloses the use of a shape memory elastomer undergoing stretch/relaxation cycles to stimulate the growth of cells. This utilization of the shape memory elastomer in stretch/relaxation cycling can be compared to a crude pump if configured properly. As described herein, a sheet or strands of shape memory material, such as this shape memory elastomer, may be configured to achieve bladder compression in a force conversion apparatus.
In general, energy conversion from one form to another is an important concept when considering optimization of usable work output. Efficient machines for energy conversion input high energy potentials of one form and convert then into a usable output form. For example, energy conversions used in every day life include: hydraulic to electric (used in hydroelectric plants); electric to mechanical (used in electric motors); thermal to mechanical (used in fossil fuel and nuclear power plants); and hydraulic to mechanical (used in brakes, clutches and actuators). The hydraulic to mechanical conversion utilizes a high pressure input potential and yields an output of usable mechanical force movement. The amount of usable output is fully dependent on the type of machine used for the conversion and/or upon the types of energies being converted. The force converter described herein involves a mechanism which converts the high-stress and small-movement potential of shape memory materials into a usable force and movement output of either a mechanical or fluid form.
As described above, force conversion in general, for example from one type of force or energy to another type of force or energy or to change the direction a force, is an important concept in optimizing work output. Conversion, for example, from hydro to electrical, from electrical to mechanical, and from nuclear to electrical, are important conversions now used in every day life. Conversion from mechanical to hydraulic and back, such as in brakes, shock absorbers, dampers and the like, is another conversion with widespread applicability. Another type of conversion, high force and small movement to low force and large movement, also has widespread uses.
A well-known example of a hydraulic force converter is a typical automobile brake. A brake system converts mechanical foot power into a large hydraulic potential and then back to a usable mechanical force for braking. It converts a low force and large movement to a high force and small movement via hydraulic conversion.
The brake is one example of using one force to act upon a hydraulic fluid so as to urge the hydraulic fluid to produce work or output. Although shape memory materials have been used in certain types of force conversion, shape memory materials have not been used in connection with hydraulic force conversion, except as disclosed in this inventor's U.S. patent application Ser. No. 07/448,250, filed on Dec. 11, 1989. However, shape memory alloys have been used in certain types of non-hydraulic actuators.
The best-mode for the present invention uses the shape memory material Nitinol. It has been selected as the best-mode because it has the most desirable work and strain capabilities at the present time; however, other shape memory materials can be used.
Previous applications of shape memory alloys have included actuators in relays such as according to Jost (U.S. Pat. No. 3,968,380), Hickling (U.S. Pat. No. 3,849,756), and Clarke (U.S. Pat. No. 3,872,415); in temperature-sensing actuators as described by Melton (U.S. Pat. No. 4,205,293) and DuRocher (U.S. Pat. No. 3,707,694); in rotary actuators such as Block (U.S. Pat. No. 4,761,955); in electro-mechanical drive actuators such as Suzuki (U.S. Pat. No. 4,736,587); in valve actuators such as Wilson (U.S. Pat. No. 3,613,732); and the like. Many of these shape memory alloy actuators have used inefficient Nitinol springs and bent Nitinol wires as the prime mover of the actuating device and not the more efficient uniaxial Nitinol wires. The use of Nitinol springs in a shape memory alloy actuator does not allow for the maximum work/volume or work/weight ratio of an actuator device. Therefore, there is a need for an actuator that utilizes the more efficient uniaxial shape memory alloy wire as an actuating element based upon the force conversion apparatus disclosed herein.
A problem occurs, however, in the use of the more efficient uniaxial shape memory alloy wires in that the uniaxial tension of the wire is constrained to a maximum critical strain limit (CSL) of 6% to 8% strain elongation for optimal shape recovery. The attainable CSL elongation of the uniaxial shape memory alloy wire becomes a severe limitation to a shape memory alloy actuator design. Any application of an actuator requiring a larger movement and lower force is thereby severely limited with the use of uniaxial shape memory alloy wire. Therefore, there is a need for a shape memory alloy actuator utilizing a force conversion apparatus that is able to convert the high stress output and small movement of a uniaxial shape memory alloy wire actuating element to a lower force and larger movement actuator output based upon the force conversion apparatus disclosed herein.
Although many types of force conversion apparatus are known, a desirable shape memory alloy actuator would need to employ a force conversion apparatus that is simple, lightweight, compact and easily made. In addition, since many actuators are used in repeating-type functions, the force conversion apparatus must be capable of rapid response and rapid cycling. Therefore, there is also a need for a shape memory alloy actuator utilizing a force conversion apparatus that is simple, lightweight, compact, easily made, and capable of rapid response and cycling based upon the force conversion apparatus disclosed herein.
As a shape memory alloy actuator would often be used in industrial applications, the actuator should be capable of withstanding a harsh environment and be relatively easy to maintain. Therefore, there also exists a need for a shape memory alloy actuator that is rugged and requires little maintenance in operation based upon the force conversion apparatus disclosed herein.
In many applications, it is desirable to control the force output of a shape memory alloy actuator. An actuator having a precisely controllable force output adaptive to many applications would be desirable. Therefore, there exists still a further need for a shape memory alloy actuator whose actuating force output is controllable based upon the force conversion apparatus disclosed herein.
Further, many times a shape memory alloy actuator will be used in an environment where electrical current potential is readily available for providing energy for operating and controlling electro-mechanical devices. A shape memory alloy actuator capable of using electric current control for its operation would be desirable for use in such a commonly found environment. Therefore, there exists still a further need for a shape memory alloy actuator which is capable of being operated and controlled by an electric current based upon the force conversion apparatus disclosed herein.
Additionally, a shape memory alloy actuator based upon the force conversion apparatus disclosed herein can be constructed to utilize inexpensive and/or plentiful energy sources such as solar energy, waste energy and/or other thermal energies such as, for example, thermal potential fluids, waste thermal industrial fluids or solar heated fluids.
It is also desirable to have a work-producing device which provides useful work from low potentials. A force conversion apparatus based upon the disclosure herein satisfies his need.
The present invention is directed to providing a force conversion apparatus which contains a shape memory material utilized in conjunction with hydraulic force conversion. The force converter of the present invention uses a shape memory material to act upon a hydraulic force conversion unit thus acting upon the hydraulic fluid contained in the hydraulic force conversion unit and allowing the hydraulic fluid to perform work. In a basic embodiment, the force converter of the present invention uses a uniaxial shape memory material strand(s), such as a length of shape memory polymer or protein or strand(s) of shape memory alloy such as Nitinol wire, and a hydraulic cylinder containing a hydraulic piston. The best-mode for the present invention uses the shape memory material Nitinol. It has been selected as the best-mode because it has the most desirable work and strain capabilities at the present time, however, other shape memory materials can be used. The piston is moved from a first position to a second position by the action of the shape memory material, which is actuated by the application of an energy potential, such as, for example, heat or an electric current, applied to the shape memory material, thus forcing hydraulic fluid from the hydraulic cylinder. The movement of the hydraulic fluid can perform work, and the hydraulic unit can be connected to any of a number of devices such as, for example, valves, brakes, actuators, poppets, or any of the myriad of other devices capable of being operated by hydraulic force or an equivalent.
SUMMARY OF THE INVENTION
The basic hydraulic shape memory material force converter of the present invention comprises a hydraulic unit containing hydraulic fluid and a shape memory material (i.e. Nitinol) connected at one end of the hydraulic unit and its other end to a fixed point or a second end of the hydraulic unit. When the shape memory material is activated, it changes from its unactivated configuration to its activated configuration and causes the hydraulic unit to change from a first position to a second position. The hydraulic fluid contained within the hydraulic unit responds to the movement of the hydraulic unit from its first position to its second position by moving from the hydraulic unit to a receiving device, where the receiving device performs work due to the motion of the hydraulic fluid. Upon the cooling of the shape memory material, the shape memory material changes back from its activated to its unactivated state, thus allowing the hydraulic unit to return to its original position, thus allowing hydraulic fluid to return to the hydraulic unit. Optionally, the change in shape of the shape memory material may allow additional or new fluid to be drawn into the hydraulic unit through one-way valves or poppets as in the case of, for example, a motor circuit.
In a first embodiment of the hydraulic shape memory material force converter of the present invention, the hydraulic unit comprises a hydraulic cylinder, a hydraulic piston, hydraulic fluid dispersed within the hydraulic cylinder, and a length(s) of shape memory material as an actuating element. The hydraulic piston is moved from a first position within the hydraulic cylinder by the action of the shape memory material which is actuated by the application of an energy potential (i.e. electrical or pH and thermal fluids). Upon moving from the first position to a second position, the piston displaces the hydraulic fluid dispersed within the hydraulic cylinder. The hydraulic fluid is displaced from the hydraulic cylinder through a conduit to a receiving device in or upon which the hydraulic fluid may perform work. Removing the actuation energy potential (i.e. heat) from the shape memory material allows the piston to return to its first position, thus allowing the hydraulic fluid to return to the hydraulic cylinder, or additional or new fluid to be drawn into the hydraulic unit. If required, a biasing device may be attached to the piston to aid in returning the piston to its first position. In this embodiment the hydraulic shape memory material is located outside of the hydraulic piston.
In an alternate embodiment of the hydraulic shape memory material force converter of the present invention, the shape memory material length is located within the hydraulic cylinder and is immersed in the hydraulic fluid. The operation of the force converter is similar to the embodiment summarized above; however, the hydraulic fluid acts as a heat sink in removing heat from the shape memory material when it is unactivated, thus allowing the shape memory material to cool.
In another alternate embodiment of the hydraulic shape memory material force converter of the present invention, a nonrigid hydraulic unit is substituted for the hydraulic cylinder and hydraulic piston combination. In this embodiment, the hydraulic fluid is dispersed within the nonrigid hydraulic unit, which can be, for example, an expandable metal bellows or an elastomeric bladder, and the shape memory material acts upon this nonrigid hydraulic unit thus forcing the hydraulic fluid from the unit.
An additional alternate embodiment of the hydraulic shape memory material force converter of the present invention is a bladder compression unit comprising a nonrigid hydraulic fluid containment unit, such as an elastomeric bladder, contained within a sheet or layer or network of strands of shape memory material. The materials which are particularly useful for this embodiment are the shape memory polymers or elastomers. As the shape memory material contracts, it constricts the nonrigid hydraulic fluid containment unit forcing the hydraulic fluid from the containment unit through an outlet port. The movement of the fluid can be used to produce work.
The hydraulic force converter apparatus of the present invention is a simple, lightweight, compact, and easily made unit which is capable of rapid response and cycling. The force conversion apparatus may be configured to have a controllable force output. Such a controllable force output may be accomplished by, for example, the hydraulic ratios of the cylinders used, the selection of the type, size and number of the prime movers (shape memory materials), and the selection of the hydraulic and cooling fluids. As shape memory materials are capable of providing resistive feedback during heating and cooling precise heating and cooling of the lengths in feedback-dependent circuits easily is obtained.
As an example, one embodiment of the present invention is configured to provide a hydraulic shape memory alloy actuator which contains a shape memory alloy actuating element utilized in conjunction with hydraulic force conversion. The actuator of this embodiment uses a uniaxial shape memory alloy wire(s), such as a Nitinol wire(s), as an actuating element. The hydraulic shape memory alloy actuator has a pair of hydraulic cylinders, each containing a hydraulic piston. The first piston is moved from a first position by the action of the shape memory alloy actuating element which is actuated by passing an electric current through it which heats it. The second hydraulic piston responds through hydraulic fluid in communication with both cylinders to the movement of the first hydraulic piston to perform work, thereby allowing the high force output and small movement of the shape memory alloy actuating wire to be converted to a lower force and larger movement actuator output. Cooling of the actuator element allows the first piston to return to its first position, the second piston being biased to move back to its rest position.
In a first subembodiment of the hydraulic shape memory alloy embodiment of the present invention, the two cylinders are concentrically mounted; while in a second subembodiment, the second cylinder is external of the first cylinder but still in fluid communication with the first cylinder.
The hydraulic conversion apparatus utilized in the actuator subembodiment of the present invention is simple, lightweight, compact, and easily made. The conversion apparatus is also capable of rapid response and cycling. The apparatus, like the shape memory alloy actuator element itself, is rugged and requires little maintenance of operation. The actuator of the present invention may also be configured in a way such that its force output is controllable based on the number of shape memory alloy actuator elements contained in the actuator, the method utilized in selectively heating the elements, or the amount of electric current passed through the elements causing their heating. In addition, the shape memory alloy actuator elements are capable of providing resistive feedback during heating and cooling, thus allowing for precise heating and cooling of the elements in feedback-dependent temperature control circuits.
It is an object of the present invention to provide a force converter which utilizes a shape memory material in conjunction with a hydraulic force conversion unit.
It is also an object of the present invention to provide a force converter which is able to convert the high force output of a shape memory material to a hydraulic fluid.
A further object of the present invention is to provide a force converter which is simple, lightweight, compact, easily made, capable of rapid response and cycling, durable, economical and easily used.
It is an additional object of the present invention to provide a force converter which has a controllable output.
Another object of the present invention is to provide a force converter which can utilize the action of any shape memory material in any configuration and especially the more efficient uniaxial configuration.
It is yet another object of the present invention to provide a force converter which can replace currently used force converters.
It is also an object of the present invention to provide an actuator which utilizes the more efficient uniaxial shape memory alloy wire as an actuating element.
It is also an object of the present invention to provide an actuator which is able to convert the high force output and small movement of a uniaxial shape memory alloy wire actuating element to a lower force and larger movement actuator output.
It is a further object of the present invention to provide an actuator utilizing a force conversion apparatus that is simple, lightweight, compact, easily made and capable of rapid response and cycling.
It is another object of the present invention to provide an actuator which is rugged and requires little maintenance in its operation.
It is yet another object of the present invention to provide an actuator whose actuating force output is controllable.
It is still further an object of the present invention to provide an actuator which is capable of being operated and controlled by an electric current.
These objects and others are accomplished by the present invention, described in detail below, which is a hydraulic force converter utilizing a shape memory alloy.